The manual doesn’t mention any accessory ports so the interface will have to be through the mic. Luckily it has the audio output so it’s going to be a single connector on the transceiver side. Not so lucky is the lack of the hardware PTT line. The mic appears to use serial communication for that and other functions. The protocol will have to be sniffed out and implemented in the software.
GX16-8 pin 1 (AF OUT) → black 3.5mm TRRS tip (RIG_AFOUT)
GX16-8 pin 2 (MIC-AN) → GX16-8 pin 4 (GND)
GX16-8 pin 3 (MIC-AP) → black 3.5mm TRRS ring 1 (RIG_AFIN)
GX16-8 pin 4 (GND) → black 3.5mm TRRS sleeve (GND)
GX16-8 pin 6 (RXD) → green 3.5mm TRRS tip (RIG_RXD)
GX16-8 pin 7 (TXD) → green 3.5mm TRRS ring 1 (RIG_TXD)
GX16-8 pin 4 (GND) → green 3.5mm TRRS sleeve (GND)